Responsive class to all the images in the content

Parsing and modifying HTML properly using the standard PHP library is quite a pain. There are many gotchas. Here’s a well documented example that will add the img-fluid class to all images in the content.

To ensure that the doctype and HTML tags are not added to the HTML fragment a solution from this StackOverflow answer is used:

$dom->loadHTML( $content ), LIBXML_HTML_NOIMPLIED | LIBXML_HTML_NODEFDTD );

Note that there are other ways of doing this which are discussed in the SO post I linked to, but this soltion has worked well for me.

The code I’ve posted below expands on this solution to deal with utf8 characters as well.

/**
 * Adds img-fluid class to images in content.
 * Fire late to affect gallery images.
 */
add_filter( 'the_content',     'add_responsive_class', 9999 );
add_filter( 'acf_the_content', 'add_responsive_class', 9999 );
function add_responsive_class( $content ) {
    // Bail if there is no content to work with.
    if ( ! $content ) {
        return $content;
    }

    // Create an instance of DOMDocument.
    $dom = new \DOMDocument();

    // Supress errors due to malformed HTML.
    // See http://stackoverflow.com/a/17559716/3059883
    $libxml_previous_state = libxml_use_internal_errors( true );

    // Populate $dom with $content, making sure to handle UTF-8, otherwise
    // problems will occur with UTF-8 characters.
    // Also, make sure that the doctype and HTML tags are not added to our HTML fragment. http://stackoverflow.com/a/22490902/3059883
    $dom->loadHTML( mb_convert_encoding( $content, 'HTML-ENTITIES', 'UTF-8' ), LIBXML_HTML_NOIMPLIED | LIBXML_HTML_NODEFDTD );

    // Restore previous state of libxml_use_internal_errors() now that we're done.
    libxml_use_internal_errors( $libxml_previous_state );

    // Create an instance of DOMXpath.
    $xpath = new \DOMXpath( $dom );

    // Get images.
    $imgs = $xpath->query( "//img" );

    // Add additional classes to images.
    foreach ( $imgs as $img ) {
        $existing_class = $img->getAttribute( 'class' );
        $img->setAttribute( 'class', "{$existing_class} img-fluid" );
    }

    // Save and return updated HTML.
    $new_content = $dom->saveHTML();
    return $new_content;
}